This invention relates to discharge of sulfur oxides trapped by an exhaust purification catalyst of an internal combustion engine.
Tokkai Hei 9-32619 published by the Japanese Patent Office in 1997 discloses a catalyst that has a function to trap nitrogen oxides (NOx) in the exhaust gas when fuel mixture having a lean air-fuel ratio is burnt in an internal combustion engine. The catalyst reduces the trapped nitrogen oxides when air-fuel mixture having a stoichiometric air-fuel ratio or rich air-fuel ratio is burnt.
The catalyst also traps sulfur oxides (SOx) as well as nitrogen oxides. When the amount of sulfur oxides trapped in the catalyst becomes large, storing capacity of nitrogen oxides becomes small, so it is necessary to occasionally discharge the trapped sulfur oxides. The trapped sulfur oxides are discharged when the temperature of the catalyst becomes high.
In order to raise the catalyst temperature, the prior art discloses performing an additional fuel injection in the exhaust stroke of the engine such that part of the injected fuel in the additional injection is burnt in the combustion chamber and the exhaust passage of the engine, and causes the exhaust temperature to rise.
The remainder of the fuel in the additional injection is directly burnt on the catalyst. By increasing the catalyst temperature in this way, the catalyst. is made to discharge the sulfur oxides.
Tokkai Hei 10-169488 published by the Japanese Patent Office in 1998 discloses increasing of the catalyst temperature by accumulating an air-fuel mixture having a rich air-fuel ratio around a spark plug.
Hydrocarbons (HC) and carbon monoxide (CO) which are produced by incomplete combustion of the rich air-fuel mixture are reacted with oxygen in the later part of the power stroke of the engine, thereby increasing the exhaust temperature, and this increases the catalyst temperature.
When fuel is burnt directly on the catalyst, the catalyst temperature rises sharply, and to control the temperature of the catalyst within a range which does not impair the durability of the catalyst while discharge of sulfur oxides is being performed, the amount of fuel in the additional injection needs to be precisely controlled.
In general, in a fuel injector, the precision of the injection amount is poorer the smaller the injection amount, and the additional injection amount is much smaller than the main injection amount. In the method of Tokkai Hei 9-32619, therefore, it is difficult to control the injection amount precisely enough for catalyst temperature management.
Tokkai Hei 10-169488 sets the air-fuel ratio of the air-fuel mixture surrounding the spark plug very rich so that it easily causes a misfire. In this condition, the air-fuel mixture can only ignite when the leading edge of the fuel spray reaches the spark plug.
That is, the ignition timing at which the air-fuel mixture can fire is very much restricted, and any shift in the ignition timing causes a misfire, and if a misfire occurs when discharge of sulfur oxides is being performed, a large amount of fuel will flow into the fully activated catalyst, and the durability of the catalyst will be impaired.
It is therefore an object of this invention to perform sulfur oxide discharge from a catalyst without affecting the durability of the catalyst.
It is a further object of this invention to facilitate discharge of sulfur oxides from the catalyst.
In order to achieve the above objects, this invention provides a catalyst temperature control device for use with an internal combustion engine. The engine comprises a combustion chamber, a fuel injector which injects fuel into the combustion chamber, a spark plug which ignites an air-fuel mixture inside the combustion chamber, an exhaust passage which discharges exhaust in the combustion chamber, and a catalyst provided in the exhaust passage for trapping nitrogen oxides in the exhaust from a fuel mixture of a lean air-fuel ratio and reducing trapped nitrogen oxides in the exhaust from a fuel mixture of an air-fuel ratio other than the lean air-fuel ratio. The catalyst also traps sulfur oxides in the exhaust when a catalyst temperature is less than a predetermined temperature, and discharges trapped sulfur oxides when the catalyst temperature is greater than the predetermined temperature. The catalyst temperature control device comprises a sensor which detects an engine running state and a microprocessor programmed to determine whether or not a condition for discharging the sulfur oxides trapped by the catalyst is satisfied based on the running state, and control a fuel injection amount and a fuel injection timing of the fuel injector, when the discharging condition is satisfied, so as to generate a stratified air-fuel mixture in the combustion chamber. Herein, the stratified air-fuel mixture comprises a first layer surrounding the spark plug, and a second layer situated outside the first layer. The first layer comprises an atomized air-fuel mixture of a rich air-fuel ratio within an ignitable range, and the second layer is leaner than the first layer
This invention also provides a catalyst temperature control device comprising a mechanism for detecting an engine running state, a mechanism for determining whether or not a condition for discharging the sulfur oxides trapped by the catalyst is satisfied based on the running state; and a mechanism for controlling a fuel injection amount and a fuel injection timing of the fuel injector, when the discharging condition is satisfied, so as to generate a stratified air-fuel mixture in the combustion chamber. The stratified air-fuel mixture comprises a first layer surrounding the spark plug and a second layer situated outside the first layer. The first layer comprises an atomized air-fuel mixture of a rich air-fuel ratio within an ignitable range, and a second layer is leaner than the first layer.
This invention also provides a catalyst temperature control method for such an internal combustion engine that comprises a combustion chamber, a fuel injector which injects fuel into the combustion chamber, a spark plug which ignites an air-fuel mixture inside the combustion chamber, an exhaust passage which discharges exhaust in the combustion chamber, and a catalyst provided in the exhaust passage for trapping nitrogen oxides in the exhaust from a fuel mixture of a lean air-fuel ratio and reducing trapped nitrogen oxides in the exhaust from a fuel mixture of an air-fuel ratio other than the lean air-fuel ratio. The catalyst also traps sulfur oxides in the exhaust when a catalyst temperature is less than a predetermined temperature, and discharges trapped sulfur oxides when the catalyst temperature is greater than the predetermined temperature. The control method comprises detecting an engine running state, determining whether or not a condition for discharging the sulfur oxides trapped by the catalyst is satisfied based on the running state, and controlling a fuel injection amount and a fuel injection timing of the fuel injector, when the discharging condition is satisfied, so as to generate a stratified air-fuel mixture in the combustion chamber. The stratified air-fuel mixture comprises a first layer surrounding the spark plug and a second layer a situated outside the first layer. The first layer comprises an atomized air-fuel mixture of a rich air-fuel ratio within an ignitable range, and the second layer is leaner than the first layer
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.